The sensitivity of Bacteroides nodosus, the causative agent of sheep foot rot, to 24 selected antimicrobial agents was tested. Many contaminants ordinarily associated with foot rot lesions were sensitive to lincomycin, whereas B. nodosus demonstrated resistance to this antibiotic. A concentration of 1 microgram of lincomycin per ml of basal medium optimally inhibited contaminants while allowing growth of B. nodosus. The basal medium was Eugon agar with 0.2% yeast extract and 10.0% defibrinated horse blood. Parallel inoculations of 31 foot rot lesion specimens onto basal medium and basal medium containing lincomycin (selective medium) were performed. B. nodosus was isolated from 16 of the specimens cultured on the selective medium and from only 3 of the specimens cultured on the basal medium. The concentration of agar in the standard Eugon medium was found to influence the growth of B. nodosus in the presence of lincomycin. Trimethoprim also exhibited potential selectivity for B. nodosus.
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